In recent years, portable information terminals such as cellular phones, laptop computers, and PDAs, portable music players, and portable game consoles have become increasingly smaller and more light-weight and electric vehicles with secondary batteries onboard have become widespread in full scale. With such trends, nonaqueous electrolyte secondary batteries used as driving power sources are also required to achieve ever higher capacity and longer lifetime. One way to achieve a high capacity is to increase the end-of-charge voltage compared to conventional art. However, increasing the end-of-charge voltage causes the positive electrode and the electrolyte in a charged state to react with each other, resulting in oxidative decomposition of the electrolyte. In order to increase the capacity and extend the lifetime of a nonaqueous electrolyte secondary battery by suppressing the oxidative decomposition reaction, the following proposals (1) to (3) were made.
(1) A positive electrode active material is proposed in which a core containing at least one lithium compound and a surface-treated layer formed on the core contain a coating material such as a hydroxide or the like of Mg (refer to PTL 1 below).
(2) An electrode additive is proposed in which a surface of a core particle composed of magnesium hydroxide or the like is coated with a conductive material (refer to PTL 2 below).
(3) A conductive agent constituted by fine particles coated with ITO obtained by doping tin oxide is proposed (refer to PTL 3 below).